Asymmetric digital subscriber line (ADSL) modems transmit high rate digital data over existing twisted pair telephone lines to connect residential customers to digital networks. Discrete multi-tone (DMT) is a multicarrier modulation technique used in xDSL systems for transmitting high speed data in the presence of significant inter-symbol interference. ADSL and DMT concepts are described in U.S. Pat. Nos. 5,408,260 and 5,533,008, and therein cited references, all of which are incorporated by reference herein. ADSL is well-suited for applications in which the data rate needed in both directions is not the same. Such an application is Internet access.
The Internet is gaining wide-spread use for both homes and offices. Most Internet applications use high speed transfer rates in the downstream direction from the central office (CO) to the remote unit (RU), and lower bit rates in the upstream direction from RU to CO. The transmission is full duplex; the transmit and receive happen simultaneously. Once the connection is established, it is desirable to maintain the connection even during idle periods.
For Internet applications, downstream data transmission rates at any given time depend upon current customer data download requests. This usually does not happen continuously (non-interrupt) during the connection. FIG. 1 shows the transmit portion of one ADSL modem connected through a channel to the receive portion of another modem. In this system both valid and idle data are treated the same--both forms of data pass through the entire encoding and decoding operations: crc, mux, synch, etc. Because idle data transmission conveys no information, many of the encoding and decoding operations could be eliminated with no decrease in data throughput. However, some form of filler data that does not require modulation/demodulation must be put in its place and a method of controlling system state transitions must be implemented.